U.S. Pat. No. 7,212,931 B2 discloses a known electric energy meter.
The known energy meter has two input terminals for connecting to an AC mains and two output terminals to which a load may be connected. A live line connects one of the input terminals to one of the output terminals. A neutral line directly connects the other ones of the input and output terminals.
Current is measured by installing a primary conductor of a DC tolerant current transformer in the live line. Terminals of a secondary winding of the current transformer are connected to each other via a resistor and to current detection ports of an integrated circuit. A secondary voltage induced in the secondary winding by a current flowing through the load is also present at the current detection ports of the integrated circuit.
Voltage is measured by mounting resistors in series so as to form a voltage divider between the live and neutral lines. The integrated circuit has voltage-measuring ports one of which is connected to an intermediate point between resistors of the voltage divider and the other is connected to the neutral line.
The measured current has a phase shift with respect to the measured voltage. In order to obtain a correct measurement of the energy consumption of the load, this phase shift must be taken account of in the integrated circuit.
The integrated circuit includes an analog-digital-converter and a microcomputer. The analog-digital converter operates at a frequency far in excess of the nominal mains frequency so as to obtain a first series of samples representative of the current measurements and the voltage measurements. Prior to multiplying corresponding samples of the first and second series, and accumulating the products to obtain the amount of electric power consumed in the load, the samples of the current are corrected for the phase error. To compensate the phase error caused by the current sensor compensated current samples are obtained by application of a formula:In,corr=In−(In−In−1)*tmains2*fmains,nom*fCLK*φCTnom/360°wherein (In−In−1) is the difference of two succeeding samples. The other terms are a real, i.e. non-complex, correction factor. All computations are done in the time domain. In particular compensation is done in the time domain and uses only real (non-complex) arithmetic.
There are several drawbacks of the known system. In the known system the term (In−In−1) is used as a numerical approximation of the time derivative of the current. Using numerical approximation in the compensating for phase error is a disadvantage, leading to decreased accuracy. Furthermore, the current signal is corrected in its entirety; yet, different harmonic parts of the current signal may require different correction. This is not possible in the known system, which assumes that phase error of current sensors is constant; however, they may depend on the temperature and current. Accuracy of the known system thus varies with temperature and current. The analog to digital conversion of the known system does not convert voltage and current at the same time. This introduces a further delay between voltage and current measurements. This additional error is not corrected in the known system.